Make Nightwatch optional

[mrdavidburns]

Some projects may use other testing frameworks so Nightwatch is not always required for all projects. Let's make this optional to where you would add drainpipe configuration to composer.json which would then scaffold Nightwatch into a project.

example:

    "extra": {
        "drainpipe": {
            "github": [
                "PantheonReviewApps",
                "ComposerLockDiff"
            ],
           "testing": ["Nightwatch"]
        },

[deviantintegral]

Should this issue be transferred to the drainpipe project, now that we push code here from there? I'm wondering if we should also disable issues and PRs on this repo.